NEC manual Operating Precautions for VR4133 TM, Details

Operating Precautions for VR4133 TM

No. 9	Ethernet: excessive data transfer into memory
	(Direction of usage)
	Details
	If two or more packets are received continuously, excessive data may be written at the end of a
	packet. The length of excessive data depends on the value of the DRBS0/1 bits of register
	RCV_CFGR0/1 (0x0f00 1618 / 0x0f00 1918) and becomes up to [burst length -1].

Therefore one of the following workarounds must be implemented:

(1)Set up burst size as 1 word (4 bytes) by RCV_CFGR0/1 (0x0f00 1618 / 0x0f00 1918).

(2)Allocate the size of the receiving buffer on a memory as [receiving maximum packet length] + [burst size - 1].

No. 10	Ethernet: transmit short packet
	(Direction of usage)
	Details
	If following two conditions are both satisfied, the internal DMA controller reads DMA buffer at the
	same address repeatedly and the ether MAC misunderstands it as a jumbo-frame and a transmit
	abort may occur, though the length of actual transmitted packet is shorter than the setting of
	LMAX0/1 (0x0f001414 / 0x0f001714) register:

(1)The length of transmitted packet (indicated by SIZE[15:0]) is longer than the setting of DTBS bit of XMT_CFGR0/1(0x0f001600 / 0x0f001900) register and the packet length is less than 32 bytes; or the burst size set by the DTBS bit is 8 words (32 bytes) and the descriptor buffer size is 30 or 31 bytes.

(2)The physical buffer address pointer, which is indicated by transmit descriptor is not word-aligned.

Therefore one of the following workarounds must be implemented:

(1)Set 8-word (32bytes) to the burst size of DTBS in XMT_CFGR0/1(0x0f001600 / 0x0f001900). If the descriptor size is 30 or 31 bytes, set a word-aligned address as the physical address value indicated by the transmit descriptor.

(2)Set word-aligned address to the physical buffer address pointer, which is indicated by transmits descriptor.

(3)Set 32-bytes or more to SIZE[15:0] of transmit descriptor.

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